OK, we've got some discussion going on about issues affecting life within the gray zone of the Halachic Dateline and the International Dateline, but what is the original source of such a concept, if there is one? Can it not just be that whatever is deemed Saturday in some remote place in the world is Saturday (as in the minority opinion at the end of this article)?
I'm not sure I understand your question, but basically many rabbis feel that the Torah states where the dateline should be, so we don't really care what the local population thinks. (Though a frustrated Rabbi Yitzchak Zev Brisker, when asked about the dateline, exclaimed -- " I don't know. And I don't know why everyone else thinks they know!")
There are various Talmudic statements that could be read as placing the dateline either 90 or 180 degrees east of Jerusalem. If you feel the Talmud fixes the Halachic dateline, that's what we'd be bound to follow.